Piezo-electric actuators are used in imaging systems to adjust the positions of zoom and focusing lens systems. Piezo-electric actuators are also used in other systems to move small objects. Driving devices providing suitable driving voltages are required to operate said piezo-electric actuators. Piezo-electric actuators are implemented by using one or more piezo-elements, which are deflected, expanded or contracted when coupled to an activating voltage.
An aspect in mobile devices is that the voltage of an available power supply is typically lower than the optimum driving voltage of the piezo-elements. Further, in mobile systems, the available power supply is typically a battery with a limited capacity. Therefore energy consumption related to the driving of piezo-elements is a problem especially in mobile systems. A piezo-element comprises a substantial internal capacitance, which stores energy when the piezo-element is charged by coupling to a driving voltage. In order to save energy, it is advantageous to re-use the energy stored in the piezo-elements.
It is known as such that a piezo-element may be connected in series with an inductive element to implement a resonance circuit. When such a resonance circuit is driven with a resonance frequency, the capacitive energy stored in the piezo-element is re-used. Further, thanks to the resonance, the voltage over said piezo-element is higher than the driving voltage. A device based on such arrangement is disclosed for example in a Japanese patent application JP06058847.